Thermoplastic elastomers (TPB) exhibit the functional properties of conventional thermoset rubbers, yet they can be melted repeatedly and are therefore suitable for processing in conventional thermoplastic fabrication equipment. The majority of TPE consist of two phases, one consisting of a rubber material (elastomer) that is insoluble in the other, and a flowable thermoplastic material. The rubber material is present as a dispersed phase and the thermoplastic is the continuous phase.
Although it is in principle not necessary to crosslink the rubber in a TPE, it has proven efficient using crosslinking techniques to obtain better chemical resistance, mechanical properties and a better control of phase separation. Such TPE compositions where a crosslinking reaction and process is used to achieve phase separation into divided domains are called Thermoplastic Vulcanizates (TPV). To keep their thermoplastic character, it is essential that only the rubber phase be crosslinked. For an extensive and detailed description and review of TPV technology, see for instance S. Abdou-Sabet, R. C. Puydak and C. P. Rader in Rubber Chemistry and Technology, vol.69, pp 476-493, 1996.
Furthermore, it has been demonstrated that the mechanical performance of TPVs improves with the degree of crosslinking of the rubbery phase and with the inverse of the particle size of rubbery domains. Dynamic crosslinking (which consists of intimately mixing a blend of compatible polymers, then introducing a crosslinking system in the mixture while the mixing process is continued) is used to generate the finely dispersed, highly crosslinked rubbery phase from a homogeneous blend of polymers.
For thermodynamic and hydrodynamic reasons it is preferred that the polymer viscosity be increased while crosslinking is taking place, because the particles tend to agglomerate while the phases are separating. Moreover, if a phase-inversion process can take place while crosslinking, this is favorable to the formation of fibrous rubber domains which may provide specific mechanical properties. However, it has been found preferable to select a crosslinking mechanism that can involve in part the thermoplastic phase, not to the point where the thermoplastic character of the TPV is removed, but only to achieve better adhesion and compatibility of the polymers.
The selection of a crosslinking process and chemicals is governed by processing requirements, e.g., reaction rate at the processing temperature; compatibility with the elastomer; side reactions with the thermoplastic; efficiency (number of crosslinks generated by each molecule of crosslinker); absence of undesired reactions; toxicity and hazards; color; and odor.
One example of such TPVs is EPDM/PP described in U.S. Pat. No. 3,130,535. EPDM and PP are mixed intimately in an internal mixer, and a peroxide is added to crosslink the EPDM. Excess peroxide and/or excessively high processing temperature and/or excessively reactive polymers will cause degradation of the PP phase and/or scorch. In contrast, insufficient amount of peroxide and/or too low processing temperature and/or a poorly reactive EPDM will cause insufficient crosslinking.
One deficiency of polyolefin-based TPV""s is that they cannot be painted without a preliminary surface treatment. It was disclosed in U.S. Pat. No. 4,311,628 that other crosslinking agents can be used, e.g., dimethylol octyl phenol resin and sulfur. Superior mechanical properties could be achieved, but unfortunately both systems suffer from excessive odor and/or yellowing of the resulting materials, as well as the difficult control of sulfur cure reactions.
It was disclosed in European Patent 0 324 434 to use silane-grafted polymers in the thermoplastic phase. After mixing, the material is shaped and left to react with atmospheric moisture. Thus it was possible to obtain a more elastomeric material after water cure. However the obtained water-cured item no longer contains a thermoplastic elastomer and cannot be recycled. To overcome this limitation, European Patent 0 409 542 disclosed mixing an EPR (ethylene-propylene rubber) or EPDM with a crystalline ethylene-propylene thermoplastic, an organofunctional silane and a free radical generator. The silane is grafted to the resin by the free radical generator and crosslinking takes place through reaction of the silane with water.
A refinement of the above processes is disclosed in European Patent 0 510 559 where the EPR or EPDM first is grafted, then mixed to the thermoplastic PP and to a crosslinking additive comprising water. The same process is disclosed using very low or ultra low density polyethylene (VLDPE or ULDPE) to reduce raw materials costs and lower mixing temperatures. See DE 44 02 943. It is also suggested to add simultaneously the PP component and the PE component together with silane and radical generator as a dry compound, the addition of water and condensation catalyst being made in a subsequent stage. However, the addition of water into an extruder at temperatures well above its boiling point is a difficult process. Moreover, the amount of water needed is so low that its metering requires sophisticated instrumentation in contradiction with the aim of the patent.
U.S Pat. No. 4,146,529 to Yamamoto et al. discloses reacting an acid modified polypropylene with an amino or epoxy silane, but the purpose of such reaction is to use the alkoxy groups to bind to fillers and to react the non-grafted carboxylic anhydride to form low odour, non-volatile products, not to crosslink the alkoxy functionalities amongst themselves. The intent of these compositions is to couple mineral fillers and not to form thermoplastic vulcanizates; or, in the absence of filler, to favor reaction of the amino or epoxy of the silane with free, volatile, non-grafted acid or anhydride.
German Patent DE 196 29 429 teaches (amongst other issues) the use of pre-blends of vinyl silanes, amino silane and unsaturated carboxylic anhydrides which are used for crosslinking of polyolefins respectively.